1. Field of the Invention
The present invention relates to an external counterpulsation cardiac assist device which may apply positive and/or negative relative pressure to one or more limbs of a patient and, more particularly, to such device having a housing which may be used for applying positive and/or negative relative (to atmospheric) pressure to the limbs in counterpulsation with heart function, which is adapted to be assembled in situ to provide customized fit and which may use a relatively small amount of gas.
2. Description of the Related Art
A method of assisting the circulation without invading the vascular system by the external application of intermittent pressure to the body has been known. Studies have shown that application of a positive relative pressure pulse to the lower extremities during cardiac diastole can raise the diastolic pressure by 40% to 50% while the application of negative relative pressure (vacuum), during cardiac systole can lower the systolic pressure by about 30%. Hereinafter, by “relative” pressure, it is meant relative to the atmospheric (gauge) pressure.
This externally applied positive and negative relative pressure increases the venous return to the heart because of the unidirectional valves in the peripheral venous bed. In cariogenic shock accompanied by myocardial ischemia, the increased coronary flow may improve cardiac function and thus indirectly affect the hemodynamic response to this procedure. It is further believed to promote the growth of collateral channel blood vessels feeding heart tissue and to reduce the symptoms of angina.
The therapeutic results of the above-mentioned method have been well documented. However, as a practical matter, an apparatus previously used to externally apply positive and negative relative pressure to the limbs has been extremely inefficient and therefore the procedure has not found wide acceptance. More specifically, such apparatus employed for this purpose included a prefabricated hinged conical metal housing or shell housing. Within the housing, a hollow cylindrical inflatable rubber balloon-like tube was placed, within which the limb segment was situated. The balloon-like rubber tube was filled with water, which was pressurized to inflate the tube, thereby filling the interior of the housing and applying pressure to the surface area of the limb segment. To apply negative relative pressure, the water was first pumped out of the rubber tube, leaving an air gap between the rubber tube and the limb. An impermeable, rubber-like coated fabric was placed around the exterior of the housing, and was sealed around the limb to trap the air between the limb and the rubber tube. By pumping out the air trapped within the sealed fabric, the fabric first collapsed around the housing, and then negative pressure began to form within the gap between the limb and the rubber tube.
This previous apparatus or system had numerous operational difficulties. Due to high resistance to flow, pressurizing the rubber tube and pumping the water out of the rubber tube fast enough to match the heart beat was very difficult if not nearly impossible. As the result, even the process of applying positive relative pressure was very difficult. The process was made even more difficult since a prefabricated housing could not be made to closely fit every patient. As a result, a relatively large gap may have been left between the rubber tube and the limb to be filled by the expanding rubber tube. In such situation, the amount of air that had to be pumped out of the rubber-coated fabric enclosed space around the housing and in between the limb and the rubber tube was relatively large, thereby requiring large air pumping action. In addition, due to the flexibility of the rubber-coated fabric, it would tend to deform and enter the space between the limb and the rubber tube, thereby making it difficult to achieve the desired level of negative pressure (vacuum) around the limb.
Other apparatus or applicators may utilize a prefabricated and relatively non-extensible fabric within which a balloon-like element is located. The balloon-like element with its enclosing housing or cuff is wrapped around the limb and secured by straps equipped with hook and loop tape, commercially known as VELCRO. Such applicators may be or may have been supplied from Vassmedical, Inc. of Westbury, N.Y.
During operation, the balloon is pressurized by air, thereby applying pressure to the surface of the enclosed limb. Due to the bulging and deformation of the cuff as the balloon is pressurized, a relatively large volume of air is required to achieve the required limb surface pressure. This is the case even though the cuff material is relatively non-extensible and the cuff may be applied snugly to the limb segment. As the result, large capacity pumps are required to drive the apparatus because of the large volume of air which has to be rapidly moved in and in most cases out of the balloons, to alternatively inflate and deflate the balloons, to apply the required pressure to the limb. Additionally, this applicator and all variations thereof that use balloons to apply pressure, cannot be used to apply relative negative pressure to the limb. Another disadvantage of such applicators is that due to the requirement of a large air volume, the system is rendered non-portable, and hence cannot be made available outside a fixed treatment room and may not be available in emergency situations.
An attempt was made to develop design concepts with a rigid or semi-rigid outer shell which surround an inflatable balloon-type interior. An applicator of this type is illustrated in U.S. Pat. No. 5,554,103 issued Sep. 10, 1996 to Zhang, et al. and U.S. Pat. No. 5,997,540 issued Dec. 7, 1999 to Zhang, et al., both of which are owned by Vasomedical, Inc. of Westbury, N.Y. Those applicators are described to be wrapped around the limb and held in place with some means such as straps of VELCRO. However, such prefabricated applicator designs cannot closely fit the limb and thus still require a large volume of air to provide the required limb surface pressure level. This is the case since such prefabricated applicators cannot be made to precisely fit a limb segment, thereby leaving a significant dead space between the balloon-like tube and the limb.
The aforementioned patents propose to fill the dead space by spacers to reduce the amount of air required for the operation of the applicator. These spacers have to be cut in various shapes and thicknesses and therefore are highly cumbersome and impractical.
The outer shells and applicators may be custom made to fit the limb segments. A large number of applicators of various sizes and shapes may also be fabricated to nearly accommodate the contour of the limbs of various patients. As is to be appreciated, custom made applicators may be impractical. As also is to be appreciated, fabricating and/or maintaining an inventory of a large number of applicators of different sizes and shapes suitable for a wide variety of different size patients such as for a hospital may also be impractical.
In addition, since such applicators may operate by pressurizing balloon-like tubes around the limb segment, they cannot be used to apply negative relative pressure to the limb segment.